1. Field of the Invention
The present invention relates to a pole-piece unit, a method for assembling the same, and a magnetic field generator. More specifically, the present invention relates to a pole-piece unit, a method for assembling the same, and a magnetic field generator used for an MRI apparatus and so on which requires an intense magnetic field greater than 0.3 T.
2. Description of the Related Art
In this kind of large magnetic field generator, e.g. a magnetic field generator for an MRI, in order to reduce magnetic flux leakage from pole pieces and to concentrate the magnetic flux effectively onto a space between the pole pieces, the applicant of the present invention proposed an arrangement, as disclosed in the Japanese Utility Model (of examined Application for opposition) No. 2-49683, in which a permanent magnet for repelling the magnetic flux leakage is provided on an outside surface of an annular projection. When an intense magnetic field is required in this related art, a Rxe2x80x94Fexe2x80x94B magnet which has an intense magnetic power is used as the permanent magnet. In this case, the permanent magnet to be bonded comes under an intense repelling force when brought near the permanent magnet which is already bonded. Thus, it is difficult to bond the permanent magnet to the annular projection which is formed as a single piece, resulting in a poor production efficiency.
Further, according to the above related art, the permanent magnet for repelling the magnetic flux leakage is a magnet magnetized in a direction in parallel to a main surface of a permanent magnet used for generation of the magnetic field. If a uniform space is to be enlarged, the amount of magnet must be increased, causing a problem of increased cost.
It is therefore a primary object of the present invention to provide a pole-piece unit, a method for assembling the same, and a magnetic field generator for improved production efficiency.
Another object of the present invention is to provide a magnetic field generator in which the uniform space can be increased without increasing the cost.
According to an aspect of the present invention, there is provided a pole-piece unit comprising: a plate yoke; a permanent magnet group provided on a main surface of the plate yoke; and a pole piece provided on a main surface of the permanent magnet group; wherein the pole piece includes an annular projection having a plurality of annular-projection pieces, and a permanent magnet for reduction of magnetic flux leakage provided on an outside surface of each of the annular-projection pieces.
According to another aspect of the present invention, there is provided a method for assembling a pole-piece unit, comprising: a first step of fixing a permanent magnet onto an outside surface of an annular-projection piece; and a second step of fixing the annular-projection piece already mounted with the permanent magnet onto a main surface of a base plate.
According to this invention, the permanent magnet is fixed in advance onto the outside surface of each annular-projection piece, and a predetermined number of the annular-projection pieces each mounted with the permanent magnet are installed onto the base plate. Therefore, there is no need for installing the permanent magnets to the annular projection which has been disposed on the base plate. Thus, fixation of the permanent magnets becomes easy, and assembling of the pole-piece unit becomes easy, making possible to improve productivity in manufacture of the pole-piece unit.
According to another aspect of the present invention, there is provided a magnetic field generator comprising a pair of the pole-piece units, and a column yoke magnetically connecting the pair of the pole-piece units.
According to this invention, by using the above pole-piece unit which can be assembled easily, the magnetic field generator is obtained easily, and productivity is increased.
Preferably, in the above method for assembling the pole-piece unit, the annular-projection piece has a guide hole; and the second step includes a step A of attaching a guide stake to the base plate, and a step B of guiding the annular-projection piece already mounted with the permanent magnet to the main surface of the base plate, by inserting the guide stake into the guide hole. In this case, the annular-projection piece can be guided to a desired place on the base plate, and therefore, positioning of the annular-projection piece becomes easy and the assembling of the pole-piece unit becomes easy.
According to still another aspect of the present invention, there is provided a method for assembling a pole-piece unit including an annular-projection piece having a flat outside surface, wherein a permanent magnet is slid on the outside surface for fixation of the permanent magnet to a desired position on the annular-projection piece.
According to this invention, since the outside surface of the annular-projection piece is a flat surface, the permanent magnet can be slid linearly and easily on the outside surface. Therefore, the permanent magnet can be easily transported and fixed to a desired position on the outside surface of the annular-projection piece. Thus, the assembling of the pole-piece unit becomes easy, and productivity in the manufacture of the pole-piece unit is improved.
According to still another aspect of the present invention, there is provided a magnetic field generator comprising: a plate yoke; a permanent magnet group provided on a main surface of the plate yoke; and a pole piece provided on a main surface of the permanent magnet group; wherein the pole piece includes an annular projection and a permanent magnet for reduction of magnetic flux leakage provided on an outside surface of the annular projection, and the permanent magnet for reduction of magnetic flux leakage is disposed, with a magnetizing direction thereof being slanted with respect to the main surface of the permanent magnet group.
According to this invention, by disposing the permanent magnet for reduction of magnetic flux leakage, with the magnetizing direction slanted with respect to the main surface of the permanent magnet group, greater amount of magnetic flux can be introduced into an imaging space, and the uniform space can be enlarged. Therefore, the uniform space of magnetic field strength can be enlarged without increasing the amount of permanent magnet to be used, and without increasing cost.
Preferably, the permanent magnet for reduction of magnetic flux leakage is generally rectangular parallelepiped, and the magnetizing direction makes an acute angle with respect to a main surface of the permanent magnet. In this case, by only disposing the permanent magnet for reduction of magnetic flux leakage directly onto the main surface of the permanent magnet group, disposition of the permanent magnet for reduction of magnetic flux leakage having a desired magnetizing angle can be carried out easily.
Further, preferably, the permanent magnet for reduction of magnetic flux leakage is generally rectangular parallelepiped, and is slanted with respect to the main surface of the permanent magnet group. In this case, a permanent magnet magnetized in any direction can be used as the permanent magnet for reduction of magnetic flux leakage.
Further, preferably, a dimension of the pole piece is determined in accordance with a magnetizing angle of the permanent magnet for reduction of magnetic flux leakage. By adjusting the size of the pole piece in accordance with a magnetizing angle of the permanent magnet for reduction of magnetic flux leakage, a center magnetic field strength can be increased without increasing the amount of permanent magnet to be used and without increasing cost. Especially, it is more preferable if the magnetizing angle of the permanent magnet for reduction of magnetic flux leakage is greater than 0 degree and smaller than 60 degrees.
It should be noted here that in this specification, the term xe2x80x9cmagnetizing anglexe2x80x9d means an angle made by a main surface of the permanent magnet group and a direction in which the permanent magnet for reduction of magnetic flux leakage is magnetized. The term xe2x80x9cuniform spacexe2x80x9d means a magnetic-field space in which the magnetic field uniformity is within 100 ppm. The term xe2x80x9ccenter magnetic field strengthxe2x80x9d means a magnetic field strength at a center portion of the uniform space.
The above objects, other objects, characteristics, aspects and advantages of the present invention will become clearer from the following description of embodiments to be presented with reference to the accompanying drawings.